Many forms of communication rely on a high degree of synchrony between transmitter and receiver to convey information. The examples are numerous: coherent FM receivers utilize phase locked loops, direct spread spectrum techniques are based upon modulating and demodulating a baseband signal with a synchronized chip sequence, optical links feature clock and data recovery receive circuitry, and likewise ultra-wideband (UWB) radio relies on receiver and transmitter synchrony.
Ultra-wideband (UWB) radio is a method of RF/wireless communications utilizing short duration pulses instead of a continuous wave sinusoid to transmit information. FIG. 1 shows the difference between a continuous wave signal 12 and an UWB signal 14. Whereas continuous wave signal 12 is constantly active both during data transmission 11 as well as after 13. UWB signal 14 is only active during pulse transmissions 16, which allows the R.F. front end to be turned off during inactive periods 18.
It is well known that the time-limited, wide spectrum signaling in UWB promises greater network capacity over traditional radio architectures, allowing superior data-rate and spatial capacity at similar power consumption over short distances. The short pulse signaling also allows duty cycling of the RF front end to save power. However, achieving these benefits of ultra-wideband communications is contingent on precise synchronization between transmitter and receiver such that transmitted pulses are received. For instance, if a transmitter and receiver are not synchronized to the same clock and a pulse is transmitted, the receiver may not be active and miss the data. However, if the two are synchronized together, then the receiver will be able to capture the pulse even as the receive duty cycle is reduced.
A popular practical implementation of synchronization is in the use of a high speed DLL/PLL in conjunction with a digital pulse tracking backend that maintains synchronization throughout the period of communications. The drawback of this approach is that the receiver and transmitter clocks must have center frequencies matched on the order of ten to hundreds of parts per million to maintain adequate synchronization, thereby necessitating that the local oscillators of both the transmitter and receiver be referenced to well matched crystals so that frequency drift between them is minimized. This requirement for a crystal imposes a significant cost to a system that a manufacturer would ideally like to avoid.
A popular method of UWB signaling is time hopping for low to medium pulse rates on the order of hundreds of KHz to the low hundreds of MHz. The time hopping method of UWB transmissions is based on a transmitter sending time limited pulses of data at times known by the receiver, which looks at the received signal at the agreed-upon times and determines the data that was sent. FIG. 2 shows a popular manifestation of the above method which divides each UWB transmission packet 20 into frames 22 and then further subdivides those frames 22 into bins 24. Many frames 22 compose a packet 20, while many bins 24 compose a frame 22. Within each frame 22, there can be only one transmission of an UWB data pulse. This transmission will fall into a certain bin 24a. The bin 24a that the pulse falls in will be determined by a template sequence that is common to both receiver and transmitter. Thus a receiver with the same template sequence as the transmitter will know the appropriate bins over which to look for the data, while pulses from other transmitters will fall in other bins where they are ignored. Synchronization is vital in this scheme because without it, the receiver cannot know when the transmitted data is valid.